Fortress Underground is a Shooter game, published by Kingsoft, which was released in Europe in 1987.
The aim of the game is to track down and destroy a hostile power station in a gigantic underground cavern with the means of a battle helicopter. After starting the game by pressing the fire button (joystick in port 1 or 2), you can direct the helicopter with the joystick in all eight directions. In addition, you can activate the cannon on board of the helicopter with the joystick fire button. Watch for the fuel supply during the flight and if low on fuel, you should land on a fuel station. Immediately after landing on the fuel station, your helicopter will be refueled. You should also fly to all stations with the sign 'STAY HERE' because you can start again from one of these stations in case you have lost a helicopter. This movement spares yourself the trouble to get back to the beginning.